For an electric motor that performs critical functions, for example in military applications, it is essential that the motor continues its operation even in the event of a failure of a phase. Therefore, comprehensive fault tolerance becomes a major aspect of the motor design. Conventionally, a motor is equipped with redundant elements to support fault-tolerant operation.
For example, U.S. Pat. No. 4,434,389 discloses a brushless DC motor having a permanently magnetized rotor and a stator with redundant sets of windings. The rotor is formed with four or more magnetic poles. Two sets of stator windings being used in a four-pole motor, three sets being used in a six-pole motor, and four sets being used in an eight-pole motor. The windings in each set are connected in wye, delta or star configuration for three-phase excitation. The switching of the currents in the individual windings in each set of windings is accomplished mechanically by a commutator, or electronically by commutation circuits coupled to individual sets of windings. Sensing of the relative position between moving and stationary portions of the motor is accomplished by independent sets of position sensors coupled independently to corresponding commutation circuits. The commutation circuits may be operated simultaneously for maximum torque, with reduced torque being available in the event of failure of one or more circuits or windings.
Hence, the redundancy is provided at the expense of complex winding patterns causing higher cost of design and operation.
Accordingly, it would be desirable to develop a control system that would provide fault-tolerant operation of a motor without redundant elements.